There are numerous electronic circuit applications where, for proper operation, it is necessary to provide precise timing or synchronization of one portion of a circuit with another. Such timing is conveniently provided by a local oscillator whose frequency is sufficiently accurate for the requirements of the circuit being timed or synchronized. Depending on the degree of accuracy required, an oscillator may be very simple and inexpensive where frequency range can have wide latitude on the one hand, or relatively complex and expensive where a high degree of accuracy (less than a few percent error) in frequency is required. It is desirable to have an oscillator which is both simple and inexpensive and which operates at an accurate frequency.
Complementary metal oxide semiconductor (CMOS) manufacturing technology is highly developed, and for many applications, is the technology of choice in designing and implementing large scale integrated circuits.
Various kinds of oscillators have been used for on-board timing of other circuitry on a CMOS integrated circuit (IC). One kind of oscillator which lends itself particularly well to implementation by CMOS technology is a ring oscillator. In this kind of oscillator there are no inductor-capacitor tuned circuits which in other oscillators are used to accurately set the frequency of operation. Instead of tuned circuits, a ring oscillator has an odd number of identical and very simple inverting stages connected in series, with an output of each stage coupled to an input of the next stage and with the output of the last stage coupled to the input of the first stage. In one embodiment each stage is an inverter having a pair of serially connected complementary metal-oxide-semiconductor (MOS) transistors whose output switches to a high level, a "1", when a low level, a "0" is applied to the input thereof, and switches to a "0" when a "1" is applied to the input thereof. The frequency of operation of this type of oscillator is determined by the speed of progression of a switching event of "1" to "0", and "0" to "1" from one stage to another around the ring, and by the number of stages.
A conventional ring oscillator implemented in CMOS technology as part of an IC chip may have its own frequency of operation within a very wide range of frequencies. Thus, by way of example, in one ring oscillator currently being manufactured as part of a CMOS IC chip, the difference in frequency of the oscillator of one chip from the frequency of the oscillator of another supposedly identical chip can be as great as 3 to 1. This wide range of frequency of operation is caused by physical and dimensional variations inherent in the manufacturing process and by normal temperature and supply voltage changes. Obviously, where precise timing of a circuit is required (e.g., frequency accuracy to within a few percent), such a wide frequency range as this is unacceptable.
It is desirable to have a CMOS ring oscillator which is relatively inexpensive and whose frequency can be set to a desired level of accuracy. The present invention provides an effective and inexpensive solution to this problem of accuracy in frequency of operation of ring oscillators.